Many times, I have been doing this for over 10 years and became a part of my life, not only a job.
I will mention the milestones as below.
1- Validate your product idea. Use surveys, just talk with your friends, the people around you. Collect some data on your potential customers' wishes. What kind of design, material, quality, function, etc. they would expect from your idea. Because when you have the idea, you usually see the good sides of it and skip validating your idea with your market. After you launch your product, you start to get feedback from your customers and you realize that some of these issues can be solved at the very beginning you just missed it!
1- Make the industrial design. 2D and 3D drawings are necessary at this point.
2- Make a prototype, via 3D printers it became so convenient these days to have a simple, fast prototype. If you are not familiar with this, I am sure someone around your friends can help you. Or you can use professional services, if you search by Google, you will find plenty of companies around you.
3- After you finish the prototype, check if it really meets your product requirements. Cross check with your friends, people around you, collect their feedback, take notes for necessary improvement points.
4- Based on all the improvements, create your updated 3D design and do the mechanical design so that your design can be manufactured in massive scale.
5- If there is no complex system, electronics in your system, just a regular contract manufacturer service will do your job.
6- You can find your own tooling supplier for creating the plastic and metal parts or your contract manufacturer can use their own tooling partners.
7- Final assembly will be done in your assigned contract manufacturer. And better you perform a quality check/outgoing product inspection for your final product. So that you won't have any bad surprise when you receive your goods.
Let me know if you have any more questions, need any support, I am happy to help with all the stages I mentioned above.